Question: The scale on a map is 9cm : 1km. If two cities are 27cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 9cm on the map corresponds to an actual distance of 1km. A distance of 27cm on the map is the same as 3 $\cdot$ 9cm. The actual distance between the two cities is 3 $\cdot$ 1km, or 3km.